If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

Re: What's a good buy in used studio lights?

We were working at a large maker of marine motors a few weeks ago. They closed their photo studio during the down turn, and they have us in to shoot some product photos a few times a year. They have pallets of old gear, some of it going back to the 1950s. For power packs, they have a bunch of Speedotron 4800 packs, which we use for photographing the big outboard motors. I plugged one in the other day, flipped the power switch, and it was like the 4th of July. Those big packs are scary when they go.

Re: What's a good buy in used studio lights?

Originally Posted by rdenney

The capacitors used are large electrolytic capacitors. They use an applied voltage to form an oxide layer over the surface of a rolled-up sheet of aluminum. The oxide layer acts as a very thin insulator, so you have a large conductor surface area with a very thin insulator separating it from another large conductor surface. The two conductors are rolled up into a cylinder and packaged into a metal can. That large surface area and thin insulator provides a LOT of capacitive reactance with respect to the package size. Capacitors work by charging a conductor on one side of an insulator, and the voltage builds as the pack charges the capacitor (the ready light bleeds a little off, so on these old packs, the ready light acts as an indicator of whether there is a charge on the capacitor--the ready light circuit also bleeds off the charge when you turn off the pack). The capacitor conductors are always wired (through the cables) to the xenon tube in the heads. When the flash is triggered, the capacitor is switched into a transformer that elevates the voltage substantially, and feeds it to a metal flash trigger in the flash head. The large voltage on that metal trigger ionizes the xenon gas in the tube, which makes it conductive, and then the capacitor has a place to dump its whole load. All that happens very quickly, of course.

(In AC circuits, capacitors act as a delay, allowing voltage changes to create charges that cause voltage changes on the other side of the capacitor, but delayed as a result of the capacitive reactance. But here, we just use the capacitor as a high-voltage static-discharge battery.)

So, with a 1600WS power pack (like my Speedotron D1600), the capacitor is big enough to hold 1600 joules, or enough energy to move a pound about 1200 feet if you turned it into kinetic energy.

What makes them fail is that the oxide corrodes away over time when the applied voltage that creates the oxide isn't there. That allows the aluminum sheets to become uninsulated, and charge arcs over across the two conductors in the capacitor. Usually, the arc causes the capacitor case to swell up and sometimes blow out. You can have leakages of voltage where the insulation is weak but not yet sufficient to cause the capacitor to arc. "Reforming" the capacitor is simply applying voltage for a while without building the flash charge so that the oxide layer can be re-established. That's why regular use keeps them in better condition than long periods of non-use. And the most likely defect is that the capacitor will arc as soon as you turn on the power switch on the head after a long period of dormancy.

In the really old days, rolled, high-reactance capacitors were bigger and used wax paper as an insulator. The wax evaporates and the capacitor can short. This can apply to some really old electronic systems like radio transmitters, but I'm not sure any of the flash power packs under discussion are within decades of being old enough to have those ancient wax-paper caps. The caps in my Speedotron pack are standard (but large) aluminum-oxide electrolytics.

I've had a flash capacitor dump into my hand. The flash ended up against the wall on the other end of the room, but it wasn't the arc that move it, it was the involuntary jerk of my hand. That was a speedlight--not in the same league as a jolt from a 1600WS power pack. But a little care with the plugs is all it takes to be safe. Being impaired or spraying liquids around a studio flash power pack: not recommended.

Rick "noting that mil-spec connectors were designed for reliability, not hot-swappability" Denney

Actually electrolytic capacitors use a separate (liquid) electrolyte as the energy-storage (via electrical polarisation) media; they do not depend merely on an oxide layer. The reason they're such high capacity because of their size is because of the ionic (full of electric dipoles) nature of the electrolyte.

The usual failure mode is the electrolyte hardening and starting to conduct, which is of course spectacular - all the energy goes into evaporating a bunch of electrolyte in the middle of a capacitor, which (tightly rolled) goes off like a firecracker.

The other failure mode is the "bulging capacitor", where electrolyte starts expanding; it's usually due to the capacitor being underrated for its load current, i.e. manufacturer was too cheap. A lot of motherboards died like this about a decade ago and one of my friends' chinese flashes went that way too. Usually they either just split and spew slightly-toxic smoking goo all over your equipment though there is a small fire risk.

It's worth noting that a few tens of Joules across your body can stop your heart. 1600J can literally explode chunks (via evaporation) out of your body on contact, so don't even think about opening a pack.

Rick's point about the mil-spec connectors is important; the pins must be completely inserted before you screw down the locknut.

Re: What's a good buy in used studio lights?

I've always had good luck with used Photogenic equipment. But, as Frank said very early in the thread, don't overlook tungsten. Only recently did I buy some inexpensive Fresnel spots at ebay, and enjoy them far more than strobes when I get the chance to shoot. For added overall intensity, I've found that using both neutral density and CTO gels on a large strobed softbox gives a nice "boost" to the spots. I confess I've only used them with digital so far, but hope to limber up some medium and large format film gear with them soon.